Responsa for Bekhorot 97:36

בנו פדוי מידי דהוה אקידושי אשה התם לאו אע"ג דנתעכלו המעות הוו קידושי

<big><b>GEMARA: </b></big>It has been stated: If one redeems his son within thirty days [of his birth], Rab said: His son is [regarded as] redeemed, whereas Samuel says: His son is not redeemed. Said Rab bah: All [the authorities concerned] agree that if he said that his son's redemption should take effect 'from now' his son is not redeemed.<span class="x" onmousemove="('comment',' The money is considered only as a gift, for there is no obligation to redeem within thirty days.');">³⁰</span> Again [if he said to the priest within the thirty days] that the redemption should take effect after the thirty days and the money is still then in existence, the son is certainly regarded as redeemed, [for is as if he had given it now].<span class="x" onmousemove="('comment',' I.e., after the thirty days.');">³¹</span> Where they differ is where [he said] after the thirty days and the money had been used [by that time].<span class="x" onmousemove="('comment',' Lit., 'The money had been consumed'.');">³²</span> [In such a case] Rab said: His son is redeemed, for this is on a par with the law o betrothal of a woman.<span class="x" onmousemove="('comment',' If a man gave a woman something and said to her: 'Be thou betrothed after thirty days' in which case the marriage is valid (Kid. 59a) .');">³³</span> There [in the case of betrothal] although the money was used, is not the betrothal yet valid?

Teshuvot Maharam